1. Field of the Invention
This invention relates to a brake system, and more particularly to a brake pressure generating device for a vehicle for applying a brake pressure to wheels of the vehicle.
2. Description of the Related Art
A conventional brake pressure generating device for a vehicle is disclosed in Japanese Patent Laid Open No. 51(1976)-114563. This brake pressure generating device for a vehicle comprises a piston slidably disposed in a cylinder body for forming a pressure chamber and connected with a brake pedal, and a spool valve disposed in the cylinder body for regulating the pressure of a pressure source. The regulated pressure regulated by the spool valve is introduced into an auxiliary pressure chamber which is located behind the piston so as to boost up the piston pressure.
In accordance with this known device, because the spool valve is exposed to the pressure in the pressure chamber and the regulated pressure at opposite ends thereof so as to regulate the pressure of the pressure source, the regulated pressure is approximately equal to the pressure in the pressure chamber (the losses attributable to spring loads being omitted). Therefore, the characteristic of the relationship between the pressure in the pressure chamber and the regulated pressure is represented by a straight line inclined at 45 degrees as shown in FIG. 3. Consequently, the regulated pressure cannot be moderately provided relative to the pressure in the pressure chamber. Further, since the characteristic of the pressure in the pressure chamber relative to the brake pedal operating force depends on only the diameter of the piston, the characteristic cannot be changed or varied according to the particular characteristics of a given vehicle.